Home – Home – Jornals – Combustion, Explosion and Shock Waves 2017 number 6

The distribution of air oxygen atoms in the oxidation products of rich mixtures of syngas with air in the flame and the under autoignition conditions at constant volume was investigated by numerical simulation using the tracer method. It was found that in rich mixtures, the oxidation of hydrogen and carbon oxide occurs in stages, which is clearly visible in the profiles of the rates of formation of H₂O and CO₂. The observed stagewise nature inevitably results in the heat release rate occurring in stages. The conversion pathways and the role of the oxygen atom of the CO molecule in the heat release in these flames were investigated.

This paper describes the mathematical simulation of an industrial membrane reactor for propane dehydrogenation in the thermodynamic conjugation with hydrogen combustion (oxidation). Due to the effective removal of hydrogen through a membrane and the heat release as a result of an exothermic reaction, the temperature of the reaction stream at the inlet could be reduced to 500 gr. The fact that the process is carried out on an industrial-level membrane reactor makes it possible to reach a propane conversion of 75% with a propylene selectivity of 97%, which exceeds the figures obtained per pass in existing industrial device at higher temperatures.

A method for performance optimization of a straight-scheme hybrid rocket motor is considered. The method is based on inserting an additional amount of an oxidizer into the solid propellant with a prescribed distribution of the oxidizer mass fraction along the solid propellant charge. An analytical dependence is derived for the oxidizer fraction distribution that ensures uniform combustion and high efficiency of the solid propellant charge.

Solution is obtained to the problem of determining the conditions of hydrodynamic stability in the presence of flow of gaseous combustion products blowing over the propellant gasification surface. The blowing flow has a low gradient along the direction of motion. Analysis of the obtained dispersion equation shows the development of hydrodynamic instability with fluctuations. The transport coefficients of natural turbulence are presented in the form of the sum of two terms: the first of these is responsible for the transfer in the absence of blowing, and the second takes into account the enhancement of transport processes in blowing. Their dependence on the initial temperature of solid propellant combustion predicts a reduction in the negative erosion effect in accordance with experimental data. Accounting for the relaxation time of the evaporation process has a stabilizing effect. In the limit of strong relaxation, this leads to oscillatory stability (in the absence of blowing), the perturbations do not grow and do not decay. However, arbitrarily weak blowing leads to instability.

A mathematical model of combustion of boron particles in a ram-rocket engine is developed. The boron combustion efficiency for one-stage and two-stage injection of air into the afterburning chamber is calculated. It is demonstrated that two-stage injection of air sometimes allows the time of complete combustion of boron particles to be significantly reduced (by a factor of 1.5-3); thus, the fuel combustion efficiency in the ram-rocket engine can be increased. The simulated results are consistent with available experimental data.

The combustion of a highly exothermic mixture of calcium chromate with aluminum and boron has been studied. It has been shown that these mixtures are able to burn in a wide range of ratios of reactants. The autowave chemical conversion is accompanied by the decomposition of calcium chromate, the chemical reaction of the decomposition products with aluminum and boron, the formation of a two-phase melt of the combustion products with its subsequent gravity separation, and crystallization of the layers. Results of the study may beuseful for obtaining chromium borides.

Self-propagating high temperature synthesis in both the layer-by-layer combustion mode and the dynamic thermal explosion mode was carried out using preliminary mechanical activation of B₄C-Ti powder mixtures in a planetary ball mill. Preliminary mechanical activation modes of reaction mixtures were determined that provide a reduction in the reaction initiation temperature to 600±20 gr. The reaction products consist of mixtures of TiC and TiB₂ with submicron grain size. The results of X-ray diffraction and electron microscopy studies of activated samples and reaction products are presented.

This paper presents an experimental study of heat and mass transfer and phase transformation in the suppression of flaming combustion and thermal decomposition of model ground, crown, and combined forest fires due to the local action of water. The experiments were carried out with typical combustible forest materials (mixture of leaves, needles, and twigs) and models of trunks and branches of trees. The conditions and characteristics of suppression of flaming combustion and the thermal decomposition of combustible forest materials were determined. It is shown that in the case of crown and combined fires, local short-term (a few seconds) action of a liquid projectile does not lead to suppression of thermal decomposition of material (only localization of flaming combustion is possible). In the case of ground forest fires, this approach may be effective with an appropriate choice of the area of the combustion zone sprayed with water and the spraying intensity and time.

This paper describes the measurement results for the shock wave propagation during an explosion of an explosive material with plastic filler. The detection was carried out with the help of high-speed analog and digital video cameras. The air shock wave was visualized by means of shadow photography in transmitted light in the region of separation from expanding explosion products and using a background oriented Schlieren method in the far region. The optical measurements of the air shock wave propagation were used to estimate a maximum pressure in the compression zone at different sections and compared with the data obtained by the pressure gauges. The calculated pressure peaks were in good agreement with the measured ones. The background oriented Schlieren method proved to be a useful tool easily introduced in usual large-scale polygon experiments.

Interaction of an expanding shock wave with a layer of particles having a rough surface is considered within the framework of the collisional model of a gas suspension. The influence of roughness on the shape of the contact boundaries in the gas phase and on the boundaries of the cloud of particles is analyzed. The development of the Richtmyer-Meshkov instability is demonstrated. Factors of particle dispersion are determined. Instability evolution is found to increase the amplitude of surface disturbances, and the development of collision dynamics favors smearing of finger-type structures. If the particle motion is essentially random, the pattern of cloud spreading is similar to that observed in experiments.

A method is proposed for calculating the mean velocity of the front of a plane detonation wave in a poorly mixed mixture of a gaseous hydrocarbon fuel and an oxidizer (oxygen or air). It is assumed that the chemical composition of the mixture exhibits periodic fluctuations in the detonation propagation direction, e.g., owing to gas charge stratification. The method is based on analyzing the functional dependence of the ideal (Chapman-Jouguet) detonation velocity on the molar fraction (normalized molar concentration) of the fuel. It is demonstrated that the mean detonation velocity can be appreciably (by 10-15%) smaller than the ideal detonation velocity. A dependence is found, which allows one to estimate the degree of mixing of the gas mixture on the basis of mean detonation velocity measurements.

A method is proposed for determining the energy release in a combustible mixture, which is based on processing the trajectory of the expanding wave from the viewpoint of the strong explosion model. The wave trajectory in the case of critical initiation of multifront detonation in a combustible mixture is compared with the trajectory of a blast wave generated by the same initiator in an inert mixture whose gas-dynamic parameters are equivalent to those of the combustible mixture. The energy release is defined as the difference between the joint energy release of the initiator and combustible mixture in the case of critical initiation and the energy release of the initiator in the case of blast wave excitation in the inert mixture. Results of experimental validation of the method by an example of a stoichiometric acetylene--oxygen mixture are presented. Noticeable deviations of the experimental profile of energy release from available model concepts are observed.

Computer modeling is used within the framework of the theory of density functional to determine the physical and chemical properties of a set of energy materials, which correlate with detonation parameters and sensitivity factors. There are two models of prediction of detonation parameters and sensitivity factors formulated for molecules and explosive crystals that satisfactorily correlate with the experimental data.

The formation of a deposit from mixture of PETN with fine-dispersed aluminum under the action of neodymium laser radiation in a free generation mode is studied. This paper also describes the efficiency of a deposit as a means to reduce the energy of laser initiation of mixtures as a function of PETN dispersion, aluminum content in the initiated mixtures and the deposit, mixture density, and diameter of the region of laser action on the explosive. The compositions of the mixtures for preparation of deposits optimal in composition, which reduce the initiation energy of the mixtures of PETN with aluminum up to 3.75 times, are determined. The mechanism of functioning of the deposit during laser initiation of the mixtures of PETN and aluminum is discussed.

Dependences of the brightness temperature of the detonation front and detonation products on detonation pressure were determined by the pyrometric method in the range of 0.7-9.4 GPa. The pressure was varied by changing the initial density of the emulsion explosive in the range 0.43-1.2 g/cm3. Polymer microballoons were used as sensitizer. The dependence of the brightness temperature in the Chapman-Jouguet plane on detonation pressure is found to be nonmonotonic. In the investigated pressure range, the measured temperature values changed from 2250 to 1830 K. A comparative analysis of the application of polymer and glass microballoons as sensitizers is performed. The obtained experimental data are compared with the calculation available in the literature.

This paper presents the results of investigation of the detonation velocity of an emulsion explosive (HE) sensitized with Expancel polymer microballoons in a wide range of initial density of 0.14-1.33 g/cm3. It is shown that when the density of the emulsion explosive is less than 0.4 g/cm3, detonation with an unstable front characteristic of liquid explosives is possible.

A nanometric HMX-based polymer-bonded explosive (PBX) is prepared by using the solution-water slurry technique. The resultant PBX is composed of 94% of HMX, 5% of fluororubber Viton, and 1% of wax. The properties of the nanometric HMX-based PBX, such as sensitivity and compressive performance, are comprehensively researched. The results shown significant improvement for the nanometric HMX-based PBX as compared to the micron-sized HMX-based PBX. The friction sensitivity, impact sensitivity, and shock sensitivity of the nanometric HMX-based PBX are obviously lower by 30, 48, and 24%, respectively. Moreover, the compressive strength and strain of the nanometric HMX-based PBX are significantly higher by 273 and 33%, respectively. Thus, both the safety and mechanical resistibility of the PBX will significantly benefit from using nanometric HMX